Note: This assumes you already have questions in a question bank within your course.
Enter the Quiz you wish to modify
Locate the Actions Menu (gear icon) from the top-right corner of the page.
Select the Edit Quiz option.
Click the Add button for the page you want to add a new question to.
Select a random question from the menu
From the popup select the category from which you want to pull.
Select the Number of random questions you want pulled from this category
Click the Add Random Question button
Repeat as desired for each category from which you want to pull questions from.